Vapor drying of semiconductor substrates with isopropyl alcohol (IPA) is widely used in the semiconductor industry. In a typical IPA vapor dryer, semiconductor substrates, wet with de-ionized water, are introduced into a quartz tank containing a cloud of vaporized IPA. The substrates are typically oriented vertically, in a front-to-back relationship. A resistive heater, mounted to the bottom of the quartz tank, is used to heat a reservoir of liquefied IPA and thus generate the IPA vapor cloud. When the warm IPA vapor comes into contact with the cool semiconductor substrates, IPA condenses out on the substrates and forms an azeotropic mixture with the de-ionized water. The azeotropic mixture has a reduced surface tension that allows it to flow off the substrates, and thus remove de-ionized water from the semiconductor substrates. Unfortunately, the semiconductor substrates are not always adequately dried by this process.
The introduction of the wet semiconductor substrates into the quartz tank causes the IPA vapor cloud to collapse or re-equilibrate through condensation. Naturally, the IPA vapor cloud is restored first at the bottom of the quartz tank, which is closest to the resistive heat source. Consequently, the azeotropic mixture, which is essential to the drying process, is formed first on those portions of the semiconductor substrate that are located near the bottom of the tank. The azeotropic mixture progresses upward on the substrates, in tandem with the restored IPA vapor cloud. Thus the semiconductor substrates are dried in a bottom to top manner. The restored IPA vapor cloud, however, does not always come into contact with the de-ionized water on the upper portions of the semiconductor substrates, and subsequently form an azeotropic mixture, before the de-ionized water evaporates, forming water spots on the upper portions of the semiconductor substrate. These water spots negatively affect semiconductor device yields and reliability. Additionally, this problem is aggravated by the ever increasing size of semiconductor substrates. Accordingly, a need exists for an IPA vapor drying process that uniformly and reproducibly dries semiconductor substrates, independent of their size.